Functions of a piston ring includes: a gas seal function to assure that the combustion chamber of an internal combustion engine is gas tight; an oil controlling function that controls thickness of a lubricating oil film formed on the surface of cylinder liner; and a heat conducting function that transmits heat from a piston, which is heated by fuel combustion, to a cylinder liner in order to cool the piston. To realize such functions, a piston ring must be always subject to tension stress that will cause it to bulge and tightly contact with a cylinder liner. As a result, a piston ring will then seal high-pressure combustion gas generated in the combustion process of an engine. The sliding surface of a top ring is, therefore, inevitably subjected to high pressure and exposed to conditions under which wear and scuffing are liable to occur.
In recent years, the operating conditions of a piston ring have tended to become increasingly severe because of the following factors. To meet with tightened exhaust gas regulations, combustion temperature and surface pressure load are enhanced. Low-viscosity lubricating oil is employed. Meanwhile, high pressure fuel injection is often employed, for example, in a common-rail diesel engine. Thus, improvement in wear resistance and scuffing resistance of a piston ring will have to be required. Also, as is known from Non Patent Document 1: Machine Mechanism Thesis No. 03-1vol. III (2003), Research of Hardness of Soot in Diesel Engine Oil, page 137 by Hidetsugu Yamamoto and two others, combustion products, which form during the combustion process and have a Vickers hardness of Hv1500 or more, reach the sliding surface between the piston ring and the cylinder liner, and cause abrasive wear.
Conventionally, hard Cr gas plating is applied on the outer peripheral surface of a piston ring. Also, the martensitic stainless steel of the outer peripheral surface of a piston ring is nitrided. This conventional method is replaced by forming a hard coating, such as ion-plating nitride or carbide.    Patent Document 1. Japanese Published Patent Application No. Sho57-57868 discloses that, by use of the PVD method, a hard coating such as TiC, TiN, chromium nitride, or the like is applied on the sliding surface of a piston ring. Allegedly, TiC or TiN reactive ion-plating coating and chromium nitride reactive sputtering coating improve wear resistance and heat resistance of a piston ring. However, TiC or TiN coating formed through reactive ion-plating and chromium nitride coating formed through reactive sputtering are poor in adhesion. Their thick coating is liable to peel. Their use has therefore been limited to certain special applications, and has not found broad utility in practice.    Patent Document 2: Japanese Published Patent Application No. sho 61-87950 is directed to an improvement of Patent Document 1, Japanese Published Patent Application No. sho57-57868. Patent Document 2 discloses that the reactive ion-plating method is applied on the sliding surface of a piston ring to form a coating having an ultra-fine mixed structure of metallic Cr and Cr nitride. Since the reactive ion-plating forms an ultra-fine mixed structure of metallic Cr and Cr nitride in the coating on a sliding surface, the adherence of coating is improved, according to the disclosure, and a thick coating can be used. Durability of the piston rings is improved to such a level that the piston rings with a Cr nitride reactive ion-plating coating have since been broadly used. However, the piston rings are used increasingly in severe conditions in diesel engines, where waste gas must be purified and fuel economy must be improved. Such conditions lead to cracking and peeling of the nitride Cr coating of a top ring of diesel engine, and countermeasures are required.    Patent Document 3: Japanese Published Patent Application No. Hei4-64 discloses that wear resistance and scuffing resistance are improved by forming an HCD ion-plating coating on the outer peripheral sliding surface of a piston ring, which coating consists of Cr, carbon, and nitrogen, and which contains from 0.1 to 14% by weight of carbon and from 0.5 to 22% by weight of nitrogen. However, there is no description at all concerning the relation of the crystal structure of the ion-plating coating to crack resistance and peeling resistance.    Patent Document 4: Japanese Published Patent Application Hei 6-248425 proposes a countermeasure to deal with the problem of the ion-plating Cr nitride coating tending to crack or peel during use in a diesel engine. It is disclosed that the ion-plating Cr nitride coating formed on the outer peripheral surface of a piston ring has from 1.5 to 20% of porosity. Thus, not only wear resistance and scuffing resistance are improved, but also cracking and peeling of the coating is greatly resisted even in more severe use. It is also disclosed that peeling resistance is further improved by forming a columnar fracture morphology of the coating and orienting the CrN (111) plane texture of crystal parallel to the outer peripheral surface. The Cr nitride coating of Patent Document 4 can be thickly formed, because it has improved resistance against defective peeling off and has improved adherence. This coating has subsequently become much used for piston rings of diesel engines, which are required to have durability. However, since a number of pores are present in the coating, coating hardness is lower than the original hardness of chromium nitride. Abrasive wear of such a coating is liable to occur in recent engines, where hard particles are generated.    Patent Document 5: Japanese Published Patent Application No. Hei 6-300130 proposes to control bias voltage and concentration of methane gas during an arc ion-plating. From 2 to 11% by weight of solute carbon is contained in the crystal structure of the thus-formed chromium nitride. The coating has a Vickers hardness of Hv 1600-2200. In that disclosure, a test carried out using a Van der Horst friction tester reveals coatings produced by that method show improved scuffing resistance and toughness compared to conventional chromium nitride coatings. Nevertheless, there are no descriptions as to how much improvement the actual piston rings have attained. Experiments performed by the inventors of the present application revealed that its toughness is unsatisfactory for using it for the outer peripheral sliding surface of a piston ring in a recent type of high-load diesel engine.    Patent Document 6: Japanese Patent No. 2692758 refers to Patent Document 1 as one of prior art and aims to achieve improvement in wear resistance, scuffing resistance, and peeling of coating. The disclosed chromium nitride ion-plating coating has a (200) texture such that the X-ray diffraction peak intensity of the CrN (200) plane of CrN type chromium nitride is at least twice as high as the same intensity of the (111) plane. Using the reactive ion-plating method to form a chromium nitride coating, metallic chromium is vaporized in nitrogen atmosphere with reduced pressure. Peeling of a coating is tested by a bench test of a gasoline engine.    Patent Document 7: Japanese Patent No.2730571 discloses a chromium-nitride type ion-plating coating, containing difficult-to-peel off columnar crystals, and a flat layer that has improved scuffing resistance, is laminated on one another. Peeling of a coating is evaluated by a bench test of a diesel engine.    Non Patent Document 1: Machine Mechanism Thesis No. 03-1vol. III (2003), Research of Hardness of Soot in Diesel Engine Oil, page 137 by Hidetsugu Yamamoto and two others.    Patent Document 1: Japanese Published Patent Application No. sho57-57868    Patent Document 2: Japanese Published Patent Application No. sho61-87590    Patent Document 3: Japanese Publication Patent Application No. Hei4-64    Patent Document 4: Japanese Publication Patent Application No. Hei6-300130    Patent Document 5: Japanese Publication Patent Application No. Hei6-248425    Patent Document 6: Japanese Patent No. 2692758    Patent Document 7: Japanese Patent No. 2730517